The present invention relates to the field of chemical mechanical polishing. More particularly the present invention relates to apparatus and methods for chemical mechanical polishing of substrates used in the manufacture of integrated circuits.
Chemical mechanical polishing is a method of planarizing or polishing semiconductor and other types of substrates. At certain stages in the fabrication of devices on a substrate, it may become necessary to polish the surface of the substrate before further processing may be performed. One polishing process, which passes a conformable polishing pad over the surface of the substrate to perform the polishing, is commonly referred to as mechanical polishing. Mechanical polishing may also be performed with a chemically active abrasive slurry, which typically provides a higher material removal rate and a higher chemical selectivity between films of the semiconductor substrate than are possible with mechanical polishing. When a chemical slurry is used in combination with mechanical polishing, the process is commonly referred to as chemical mechanical polishing, or CMP.
Prior art CMP process typically include a massive rotating platen containing colloidal particles in an alkaline slurry solution. The substrate to be polished is held against the polishing platen by a polishing head or carrier which can be moved in an x-y direction over the plane of the platen from a position near its outside diameter to a position close to its center. The platen is several times larger than the substrate to be polished. The substrate is rotated independently while pressure is maintained between the substrate and the polishing pad.
The rate of material removal from the substrate in CMP is dependent on several factors including, among others, the chemicals and abrasives used in the slurry, the surface pressure at the polishing pad/substrate interface and the net motion between the substrate and the polishing pad. Generally, the higher the surface pressure and net motion at the regions of the substrate which contact the polishing pad, the greater the rate of removal of material from the substrate. It should be appreciated that equipment capable of performing this process is relatively massive and difficult to control to the precision necessary to consistently remove an equal amount of material on all areas of the substrate.
Using a large polishing pad of CMP processing creates several additional processing limitations which lead to non-uniformities in the polished substrate. Because the entire substrate is rotated against the polishing pad, the entire surface of the substrate is polished to a high degree of flatness as measured across the diameter of the substrate. However, where the substrate is warped, the portions of the substrate which project upwardly due to warpage tend to have higher material removal rates than the remainder of the substrate surface. Furthermore, as the polishing pad polishes the substrate, material removed from the substrate forms particulates which may become trapped in the pad, as the polishing slurry dries on the pad. When the pad becomes filled with particulates and the slurry dries in the pad, the polishing surface of the pad glazes and its polishing characteristics change. Unless the user constantly monitors the removal rate of the polishing pad with each substrate, or group of substrates, and adjusts the slurry, load, position, and/or rotational speed of the polishing pad to maintain the desired material removal rate, the amount of material removed by the polishing pad from each substrate consecutively processed thereon will decrease.
The present invention provides methods and apparatus for polishing substrates where the polishing pad is a flexible membrane strip or belt (preferably continuous) which moves linearly between adjacent support rollers to provide uniform polishing of the substrate in contact with the moving membrane. In one embodiment a flexible polishing membrane has a substrate holder (polishing head), holding a substrate for polishing on a first side of the linearly moving membrane and a membrane backing member on a second side of the linearly moving membrane. The substrate holder and the membrane backing member are collectively configured to provide a set of clamping forces to urge the substrate and the first side of said membrane into contact with one another for polishing.
In one embodiment the membrane backing member is a flat surface having generally equally distributed fluid holes therein. The holes face the back of the flexible polishing membrane such that when the membrane backing member is brought into close proximity to the flexible membrane and fluid (liquid or gas) is flowing out from the holes a fluid layer is formed between the surface of the backing member and the second side of the flexible membrane (belt). Clamping forces urging the belt and backing member together are generally uniformly resisted by the intervening fluid layer which provides a nearly uniform pressure between the membrane and backing member. The uniform pressure on the backside (second side) of the membrane is substantially transferred through the membrane to provide uniform mechanical abrasion over the surface of the substrate being polished by rubbing against the first side of the membrane. The set of forces urging the substrate and membrane against one another can be varied in conjunction with, or independently of, any adjustment in the speed at which the membrane moves relative to the substrate being polished.
Preferably the substrate is fixed in the substrate holder at a location generally closely adjacent to the path of the freely moving membrane (belt). The backing member is supported by an urging member whose force can be adjusted. In one example, the force supplied by the urging member on the backing member is provided by a bellows assembly having bellows whose internal pressure is controlled to maintain a pre-set force on the back of the membrane regardless of dimensional variations in the surface of the substrate and in the thickness of the membrane belt and any liquids or slurries on its surface.
Alternately, the backing member can be held fixed while the substrate holder and substrate can be urged by an adjustable urging member whose force can be adjusted. Similar to the urging member discussed above for the backing member, the force supplied by the urging member on the substrate member is provided by a bellows assembly having bellows whose internal pressure is controlled to maintain a pre-set force on the membrane regardless of dimensional variations.
As a third alternative, adjustable urging forces can be provided to both the substrate holder and to the membrane backing member. However the balancing of such forces would have to be controlled carefully to assure that nearly central alignment of the flexible membrane between its adjacent rollers (pulleys) is maintained.
Polishing of wafers as described above is done by a belt which is generally wider and longer than the size of a single substrate (wafer). Polishing contact takes place over the whole surface of the wafer at once, as the belt is generally in contact with the full width and length of the substrate""s surface at one time. If the wafer were held stationary relative to the belt, then anomalies or imperfections in the polishing membrane (belt) would be transferred to the wafers surface. To avoid or reduce the possibility that any such anomalies would form the substrate is slowly rotated and is also oscillated from side to side to distribute the effect of any such anomalies over a larger area.
To avoid excess polishing at the edges of the substrate from the natural bowing of the flexible membrane when it is subjected to pressure from one side, a perimeter or fence ring is provided around the substrate. The perimeter ring, made of a highly abrasion resistant material such as Delrin or Ultra High Molecular Weight plastics, such as polyethylene, provide an artificial extension of the edge of the substrate. The transition between the edge of the substrate and the inside diameter of the perimeter ring is flat. The edge effect which causes additional wear at locations where the membrane bends because it is displaced from its natural course by the action of either the membrane backing member or the substrate support head, occurs only at the outer edges of the perimeter ring. The edge of the substrate is therefore insulated from edge effects by the perimeter ring which acts as a buffer.
Polishing as described herein is preferably done in a horizontal plane, but can be performed in a vertical orientation, or at any other angle where the substrate can be held for engagement and disengagement with the flexible polishing membrane.
Polishing wafer can also be done by using flexible polishing membranes which provide coverage less than the full area of the wafer. One example of such a configuration provides for a flexible polishing membrane which has a width whose dimension is less than the diameter of a substrate to be polished. The substrate is mounted in a holding fixture which faces a narrow circulating belt. The belt is moved back and forth transversely across the substrate to provide polishing of the full width of the substrate. The substrate and/or the belt rotating mechanism can be slowly rotated to further avoid the localized effect of belt anomalies or imperfections from being detected in the final finish polished substrate.
Still other polishing configurations reduce the contact area between the flexible polishing membrane and the surface of the substrate to a small fraction of the area of the surface of the wafer. A set of two or more small rollers cause a narrow belt to rotate in a belt carrier unit. The unit is then manipulated to move relative to the surface of the substrate to evenly polish each unit of area on the surface. For example when the substrate is rotating independently from the movement of the belt carrier unit, the higher surface velocity of the substrate near its circumference must be taken into account by providing a lower dwell time at the perimeter while compensating for the lower surface velocity near the center of the substrate by providing a longer dwell time for the belt carrier unit.
In another embodiment, the apparatus includes a rotating plate on which the substrate is held, and polishing arm which is located adjacent the plate and is moved across the surface of the substrate as the substrate rotates on the rotating plate. The polishing arm includes a polishing pad on the end thereof, which is preferably variably loadable against the surface of the substrate as different areas of the substrate are polished thereby. The speed of rotation of the substrate may be varied, in conjunction with, or independently of, any adjustment of the polishing pad against to control the rate of material removed by the polishing pad as it crosses the substrate. The polishing arm includes a cartridge of polishing pad material in tape form, a discrete length of which is exposed over the lower tip of the of the polishing arm to contact the substrate for polishing. The tape of polishing pad material may be moved over the polishing arm tip to continuously provide a new polishing pad surface as the substrate is processed, or may be moved to provide a discrete new section of polishing pad tape to polish each new substrate or allow the movement of the tape to move together with the arm to provide polishing. In another arm based configuration, the polishing pad may be offset from the polishing arm, and the polishing arm may be rotated over the rotating substrate to cause the polishing pad to contact the rotating substrate as the polishing pad also rotates about the axis of the polishing arm.
The mechanical abrading of the surface of a substrate being polished is performed by placing a slurry of colloidal particles on the surface of the polishing membrane to act as the agent for polishing. This slurry is messy and must be kept wet to remain fluid to avoid excessive build up of particles and the polishing anomalies that such buildups may create. Deionized water is therefore run onto the belt along with the slurry to maintain its fluid state and replenish the abrasive colloidal members. An option to a stream of de-ionized water is to run the belt (continuous flexible membrane) through a bath of fluid and/or to condition the surface of the belt by winding the path of the belt over a conditioning/idler pulley. The surface of the pulley would include a grooved surface pattern such as knurling to allow a nonuniform build-up of caked on slurry to be knocked off or distributed by the pattern (usually regular) on the surface of the conditioning idler pulley. While not presently available, a dry belt which would provide the same or a very similar abrading action would be preferred to eliminate the mess and complications associated with the use of slurry. As far as is known no dry-type continuous belts for CMP are presently available.
In CMP the chemical part of the activity is performed by providing typically an alkali (reducing) solution such as NaOH to the surface of the substrate during processing. The alkali solution causes softening of the surface of the substrate. The softened layer can then be more easily removed by the mechanically abrasive colloidal particles in the slurry. The depth of softening of the surface by the alkali solution is dependent on the time of contact between the solution and the surface. The introduction and removal of alkali solution must be carefully controlled to avoid over or under polishing the surface of the substrate. The chemical treatment provides for removal of the surface layer of the substrate to a uniform depth, rather than a strictly mechanical planarization which when planarizing substrates with high and low points takes more from high points and less from low points thereby increasing the possibility that layers of material which have been uniformly deposited over underlying undulating layers will be breached and the substrate features damaged or rendered less reliable as a result of the build up of manufacturing tolerances.
A method according to the present invention includes the nearly theoretically ideal arrangement where the surface of the substrate being processed is uniformly exposed to an abrasive agent with a uniform force between the membrane carrying the abrasive and the substrate. The method includes the method steps of: holding a substrate to be processed in close proximity to a linearly moving membrane